1. Field of the Invention
The present invention relates to an automatic method to repair circuit shorts and near-shorts present in narrow bridges or remnants of bridges between circuit lines. By applying a voltage across a pair of lines, certain electrical phenomena take place at the area of the short or near-short. The electrical phenomena thus generated induce a localized etching in either a gas-phase or liquid-phase medium depending upon which embodiment is used.
In particular, the invention comprises two embodiments:
The first embodiment covers the repair of a near-short circuit which includes two methods: (A) A dry method in a gas chamber where a high voltage (i.e. 100-500V) generates a localized discharge (micro-plasma) at the narrowest point of the gap between the two lines that generates reactive ions that etch away the unwanted metal and make the gap wider; and (B) a wet method using an etching solution whose etching rate depends sharply on temperature or electric current or both, and applying a certain voltage between these two lines so that a high field concentration is created at the narrowest point of the gap between the two lines which in turn generates an area of very high current concentration that etches away the unwanted metal and makes the gap wider.
The second embodiment covers the repair of a short circuit, which includes two methods: (A) A dry method using a gas phase which utilizes a temperature dependent chemical reaction; and (B) A wet method using a liquid phase which utilizes a temperature dependent wet etching reaction.
The repair of the short is a self-intensifying process that accelerates automatically until the bridge is broken, and then it stops automatically. At that point, the gap is in general too narrow. It is then treated as a near-short as described in the first embodiment.
2. Description of the Art
As is well known, the performance and reliability of computers and any other electronic system today depend heavily on packaging technology. As a result of the tremendous progress of the speed of the chips, the time delay of signals propagating in the wires connecting the chips has becomes the major obstacle to increasing the speed of the entire computer. Since the propagation speed of any signal cannot exceed the velocity of light in the medium, it is necessary to reduce the size of the computer as much as possible. Therefore the width and spacing of wiring has become smaller and smaller, and the resolution limit of lithographic processes has been pushed continuously. Thus, the elimination of defects in the circuits is related directly to the performance and reliability of computers and any other electronic system.
For the purpose of providing some background with respect to the two embodiments of the present invention, a description of the four kinds of defects found on the boards, cards and modules is useful. These defects comprise:
1. Circuit open. A line with an open gap or space can be detected easily by continuity testing. However, to locate the physical position of the site, usually by visual inspection with a microscope, is a time consuming and expensive operation.
2. Near open. If a portion of a line necks down so it becomes much narrower or thinner than the rest of the line it could become completely open due to excessive current or mechanical fatigue thus causing failure. (see e.g., FIG. 1(b)).
A method of repairing partially defective conductor lines is disclosed in U.S. Pat. No. 4,919,971. The defective site in the circuit comprises narrow necks or thin portions or cracks in a conductor line which is heated by passing a relatively high current therethrough. Using the thermobattery principle in an electroplating process or an electroless plating process or a pyrolytic deposition process, a conductive material is deposited at the defective site, and the partially defective conductor line is thereby repaired. A method of repairing gaps and circuit opens in a conductor line is also disclosed in the application.
3. Circuit short. This defect can be detected by testing the resistance between all pairs of lines. To fix it, the bridge between the pair of lines is visually located and manually cut off or ablated with a laser. See e.g., FIG. 1(c).
4. Near short. When the gap between two lines is too narrow, i.e., existence of an incomplete bridge, it is a potential defect which may affect long-term reliability of the computer. Presently, the near-shorts are treated by visually inspecting the entire area of the circuit, then manually removing the protrusions with a focused laser. See e.g., FIG. 1(d).
The present invention relates to the "shorts" and "near-shorts" as described above. It is clear with respect to all the defects noted above that the visual and manual processes and laser processes for inspection and repairing are very labor intensive, tedious and expensive.
There is a striking similarity between the two groups of defects, i.e., the opens and near opens on one hand, (see FIGS. 1(a) and (b)), and the shorts and near-shorts on the other hand. (See FIGS. 1(c) and (d))
A circuit short, FIG. 1(c), is formed by a conducting filament (referred to as a bridge) running from one line to another, which is similar to a near-open, FIG. 1(b). The repair process for the circuit short is to remove the filament or bridge that causes the short rather than to expand it.
A near-short, FIG. 1(d), is similar to a open circuit, FIG. 1(a), which consists of a gap in the otherwise continuous conductor. The repair process for the near-short is to expand the gap between the bridge remnants rather than to bridge it.
The electrical phenomena occurring at the shorts and near-shorts are exactly the same as in the case of opens and near-opens. That is, there is local Joule heat at the conducting filament (bridge remnants) in the near-short and there is local concentration of electrical field at the gap between the two pieces of metal in the open.
Therefore, a self-induced repair process for repairing shorts and near-shorts can utilize etching processes induced by local Joule effect or local concentration of electrical field, in accordance with the present invention, as is disclosed more fully hereinafter.